The surface ocean currents are driven by atmospheric circulation, so I cannot explain ocean currents until I explain global atmospheric circulation. Out of this will spring half the local climate phenomena that we observe, such as the relationship between Naples and New York (they actually reach about the same temperatures in the summer, but NYC is colder in winter).

We know of course that the sun heats the earth more at the equator than at the poles, so one would naturally expect rising warm air at the equator, which if nothing else happened would be pushed all the way to the poles before descending again, as seen by the figure below on the left. Our winds would then flow from the poles to the equator along the surface, with upper level winds flowing the opposite direction.

But the impish earth insists on ruining this beautifully simple pattern by rotating, the cad. If you’ve ever tried to walk across a rotating merry-go-round, you find it doesn’t work. If you do the natural thing of walking straight across, the dratted thing rotates underneath you and to a mouse cowering against the hand rails you seem to go careening off opposite to the direction of rotation. If the merry go round is rotating counter clockwise, you will seem to veer off to the right. This is called the Coriolis effect. Since from the point of view of someone above the North Pole the Northern Hemisphere is indeed rotating CCW, all objects attempting to travel across the northern hemisphere will feel themselves veering to the right with respect to the earth. Two things we should note here:

1. You won’t notice anything unless the time of travel is at least a significant fraction of the time it takes for the earth to rotate.2. The closer you are to the pole, the more it looks like a merry-go-round, while at the equator you are at 90 degrees to the plane of rotation. So the Coriolis effect goes to zero at the equator (then reverses direction and increases as you go to the South pole).

So this ruins our beautiful flow of air from the equator to the poles. It gets turned and never makes it. In fact air flowing from the poles only makes it to 30 degrees latitude before being turned and sinking. The same thing happens with air from the poles, which gets turned at 60 degrees latitude, and rises because it’s warmer. This is shown in the above figure to the right. Notice that in both the equatorial and polar cells the air is rising where it’s warm and falling where it’s cold.

But a funny thing happens in the cell between 30 and 60 degrees (inhabited by all of us except the Fjordlandians): if the air rose where it was warm and descended where it was cold, it would rise at 30 degrees and fall at 60 degrees; but this would make it bump against the air going the opposite direction from the equatorial and polar cells! So it gets driven opposite to the direction thermodynamics would dictate. In the temperate zones the prevailing wind is driven by friction, not by heat. This is one of the reasons temperate weather is so unpredictable (there are others reasons too!).

So we have wind that comes from the south gets turned east, and wind coming from the north gets turned west. These surface patterns are shown below to the right, with the return air shown to the left.

The complete flow pattern with all it’s beautiful undulations is shown in the figure below:

So the upshot is that in the northern temperate zone the prevailing wind at the surface is from the west (curving from where it descended to the south) and in the tropics it’s from the east (curving from where it descended to the north). It’s these prevailing winds that drive the ocean circulation.

In the diagram above we see the prevailing surface winds shown by purple dotted lines. The ocean currents will be driven to the west in the Tropics, curving northward a bit due to the Coriolis effect until it hits the continents and has no choice but to follow the coastline up north into the temperate zone. Here the winds begin to blow to the east, which with the help of the Coriolis effect (pushing to the right) will peel the current away from the coast and head east. But the Coriolis effect is stronger in the north then near the equator, so the current will begin to be pushed southward before it even reaches the eastern coastline, creating a more diffuse southward current as compared to the compact northward current.

So that northward current carrying warm water along the US coastline is strong and tight, deserving to be named, and so we have the Gulf stream from whence it pulls its warmth. The diffuse southward current along the European coast does not merit a name. But the strong northern current along the Chinese and Japanese coasts is called the Kuroshia current, the sister to our Gulf stream.

The same thing happens in the southern hemisphere, with a mirror reflection across the equator.

Tomorrow I'll address climate differences between Naples and New York City.

_________________Halfwise, son of Halfwit. Brother of Nitwit, son of Halfwit. Half brother of Figwit. Then it gets complicated...

Note that cyclones in the southern hemisphere rotate clockwise rather than counter-clockwise as seen in the north. the Coriolis effect is reversed since you are looking at the merry go round from the bottom. That bay to the northeast of the storm is facing into the wind and may suffer some storm surge.

Quiz: does water going down drains in the southern hemisphere also rotate CW? Why or why not?

_________________Halfwise, son of Halfwit. Brother of Nitwit, son of Halfwit. Half brother of Figwit. Then it gets complicated...

maybe a better example would be Canada and England? I think they are on the same latitude? erm maybe but Canada in Winter is brutally cold but England is temperate most of the time. I must think of an intelligent global warming question sometime (which may be quite some time)

North of 30 latitude the wind tends to blow towards the east.South of 30 latitude the wind tends to blow towards the west.

imagine two big fans hitting the atlantic north of the equator. The fan closer to the equator blows water towards the west, the fan halfway to the pole blows water towards the east. North of there is Greenland and Fjordlandians, we don't care about them.

So if you sit in a tub and paddle in front of you to the right and in back of you to the left, the water will end up going around you. The water going past one side is the Gulf stream. If the hot water flowing into the tub is the equator, the Gulf stream brings that water north.

For complicated reasons having to do with rotation and the earth's curvature, the southward current is more spread out than the tight northward current, so nobody gives it a name.

_________________Halfwise, son of Halfwit. Brother of Nitwit, son of Halfwit. Half brother of Figwit. Then it gets complicated...

Since you've just covered the general principles, could I ask for an explanation of how the winds and currents come together in Drake's Passage off Cape Horn? I often find myself trying to explain this to groups of 3rd and 4th graders on board a ship. Their questions often leave me stuttering.

Meteorology? The last time a meteor - well, a Baby Mete as we call down here - shot down into Little Forumshire, it very much put the wind up Pussy Hardcock's chickens...not to mention her pussy, Tom. Don't talk to her about meteorology -- nor the weather neither... touchy subject in some quarters...

David H wrote:Since you've just covered the general principles, could I ask for an explanation of how the winds and currents come together in Drake's Passage off Cape Horn? I often find myself trying to explain this to groups of 3rd and 4th graders on board a ship. Their questions often leave me stuttering.

Actually this one is easy, though I may not have time to get to the NYC versus Naples question.

If you look at Antartica, no landmasses interfere with the winds and wind driven currents going around it, unlike every other latitude. And it's a tight circle. So the winds go spinning merrily around as the air descends at the pole and then on leaving it gets turned to the left....and keeps going. And so they drive the water in the same merry-go-round. The south polar region is basically isolated from the rest of the planet by a spinning wall of wind and current.

_________________Halfwise, son of Halfwit. Brother of Nitwit, son of Halfwit. Half brother of Figwit. Then it gets complicated...

halfwise wrote:Actually this one is easy, though I may not have time to get to the NYC versus Naples question.

If you look at Antartica, no landmasses interfere with the winds and wind driven currents going around it, unlike every other latitude. And it's a tight circle. So the winds go spinning merrily around as the air descends at the pole and then on leaving it gets turned to the left....and keeps going. And so they drive the water in the same merry-go-round. The south polar region is basically isolated from the rest of the planet by a spinning wall of wind and current.

But that should produce east winds, right? Or am I turned around? The prevailing winds at Cape Horn are westerlies, which from your theory above seems to imply they're more influenced by the tropical air from the north than the polar air from the south. Here's a wiki-quote for Cape Horn FWIW.

The prevailing winds in latitudes below 40° south can blow from west to east around the world almost uninterrupted by land, giving rise to the "roaring forties" and the even more wild "furious fifties" and "screaming sixties". These winds are hazardous enough in themselves that ships traveling east would tend to stay in the northern part of the forties (i.e. not far below 40° south latitude); however, rounding Cape Horn requires ships to press south to 56° south latitude, well into the zone of fiercest winds.

But please deal with Mrs Figg's NY vs Naples question first. I have more sense than to cut into the queue ahead of her!

Hmm, you're right...I was thinking of Cape Horn as pushing into the 60's, but it doesn't quite make it, as my map clearly shows, and the temperate zones do extend beyond the 60's depending on the season (sun isn't always over the equator). I expect ships would only attempt to round the Horn in southern summer, is that right? But I would think in winter they may be in the polar cell and have the wind at their backs depending on the direction they go. Is the wind known to switch direction? I may have to look that up.

One might expect as the zones switch from east to west the winds would go to zero (often happens in the tropical to temperate boundary called the Horse latitudes) but in the switch to the polar zone strong temperature gradients occur, and these cause pressure differences. This 'thermal wind' is a topic I can take up later; not sure but I think this may be the explanation for the strong winds in the 60's.

_________________Halfwise, son of Halfwit. Brother of Nitwit, son of Halfwit. Half brother of Figwit. Then it gets complicated...

Okay Dave, it seems like from all I can fathom the only reason for strong winds around the tip of Cape Horn is the complete absence of land at that latitude. Holds both for the westerlies and the easterlies further south. Easy enough?

_________________Halfwise, son of Halfwit. Brother of Nitwit, son of Halfwit. Half brother of Figwit. Then it gets complicated...